JP2015-22802A discloses that, in fuel cells that are adjacent, an anode separator of one fuel cell is welded to a cathode separator of the other fuel cell to join both separators. The separator includes a plurality of fluid flow passages formed such that a bottom portion that abuts on a membrane electrode assembly, and a protrusion that protrudes from this bottom portion are repeatedly arranged. The adjacent two separators are joined such that the protrusions are welded to one another.